Power control in mobile radio communications systems

ABSTRACT

A power control unit ( 500 ) for a power control system in a mobile communication system, the power control unit ( 500 ) comprising an inner power control loop element ( 503, 507 ), which generates a transmit power control command ( 504 ), and an outer power control loop element ( 516 ) connected to the inner power control loop element ( 503, 507 ), the outer power control loop element ( 516 ) being configured for providing a target value ( 506 ) to the inner power control loop element ( 503, 507 ). The outer power control loop element ( 516 ) comprises a soft information estimator ( 509 ) connected to at least one outer loop regulator ( 501, 502 ), wherein the soft information estimator ( 509 ) is configured to provide a soft information estimate ( 510 ) to the at least one outer loop regulator ( 503 ).

This patent application claims the benefit of priority from U.S.Provisional Patent Application Ser. No. 60/346,745 filed on Jan. 7,2002. This application incorporates by reference the entire disclosureof U.S. Provisional Patent Application Ser. No. 60/346,745.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to a power control system and anassociated method for a mobile radio communications system and morespecifically to a power control unit for the power control system. Thepower control unit comprises an inner power control loop element, whichgenerates a transmit power control command, and an outer power controlloop element connected to the inner power control loop element. Theouter power control loop element is configured for providing a targetvalue to the inner power control loop element.

DESCRIPTION OF RELATED ART

In mobile radio communication systems, power control is employed tocompensate for variations in a wireless propagation channel.

Power control aims at compensating for the variations in the wirelesspropagation channel to maintain a high transmission quality in themobile radio communication system. A transmitted signal is sent from atransmitter over the wireless propagation channel to a receiver. Thesignal is affected and corrupted by the channel. For example, the powerof the signal is decreased with increasing propagation distance and thesignal power is varying due to fading in the wireless propagationchannel. A power control system regulates the transmitted signal powerto a minimum power level that gives an acceptable performance at thereceiver. It is important that this level is the lowest needed in orderto have a low co-channel interference, which gives a better capacity,which for example may be measured as the number of possible simultaneoususers per unit area in the mobile communication system. Also, lowtransmitted power results in low power consumption, which is ofimportance especially to handheld and portable mobile units, due tolimited battery capacity. However, some margin to the minimum level isrequired due to the fading dips that occur in the wireless propagationchannel. Power control could be used in one of, or both, uplink (thepropagation direction from a mobile unit to a base station) and downlink(the propagation direction from the base station to the mobile unit). Inthe uplink the mobile unit is the transmitter and in the downlink thebase station is the transmitter.

Multipath fading is due to reflections of a propagating radio signalsent from a transmitter to a receiver. It causes a received signal powerlevel to vary very rapidly with deep dips now and then. To compensatefor this effect, a closed power control loop is used. The received poweris measured at regular intervals and after each measurement controlcommands are sent to the transmitter on the other end, with instructionshow to adjust the transmitted power. The transmitted power may bechanged in steps of, for example, 1 dB.

In, for example a 3GPP (Third Generation Partnership Project) solutionfor a WCDMA (Wideband Code Division Multiple Access) system, the closedpower control loop comprises an inner power control loop and an outerpower control loop. In the 3GPP specification number 25.214, “Physicallayer procedures (FDD)”, power control for a WCDMA system is described.This documentation is hereby incorporated by reference. The purpose ofthe outer power control loop is to set and continuously adjust a targetvalue for a quality measure for the inner power control loop to aim at.The target value is based on an estimated received quality measure. Aflowchart for a general prior art outer power control loop method isshown in FIG. 1.

The received quality is compared to the required quality 101. If thereceived quality is better than the required quality, the target valueis decreased 102 and if the received quality is worse than the requiredquality, the target value is increased 103.

In FIG. 2 an illustration of an uplink scenario in a prior art mobilecommunication system 200 is shown. The system comprises a radio networkcontroller (RNC) 201, which comprises an outer power control loop unit202. The system further comprises a base station 203, and a mobile unit204. The RNC 201 processes received signal information 205 from the basestation 203, and the quality measure for the received signal is input tothe outer power control loop unit 202. The outer power control loop unit202 then sets the target value 206, which is sent to the base station203 for processing in an inner power control loop (not shown) into apower control loop command 207, which is sent to the mobile unit 204. Ina corresponding downlink scenario, the power control would be present inthe mobile unit instead, thereby controlling the transmitted power fromthe base station.

FIG. 3 shows a schematic block diagram for a prior art power controlunit 300 in either uplink or downlink in a WCDMA mobile communicationsystem based on the 3GPP specification. The power control unit 300 isusually located in a receiver in the mobile communication system. Thepower control unit comprises a first regulator 301, which is a part ofan outer power control loop element 312, and a second regulator 302,which is a part of an inner power control loop. The regulators arearranged in cascade. The inner power control loop sends a transmit powercontrol command 303 (TPC) to a transmitter (not shown) to inform how itshould adjust its transmitted power. The power is adjusted in apredetermined manner. The adjustment is dependent on the estimatedquality measure 304 compared to the target value 305. If the estimatedquality measure 304 is below the target value 305, a TPC command 303 issent to the transmitter to increase its power, and if the estimatedquality measure 304 is above the target value 305, a TPC command 303 issent to the transmitter to decrease its power. The estimated qualitymeasure 304 may be a signal-to-interference (SIR) value, which a SIRestimator 306 estimates based on pilot bits 307 sent from thetransmitter to the receiver. Since the pilot bits 307 are known at thereceiver and have experienced the same propagation conditions on thewireless propagation channel as the information signal, an estimate onthe quality measure 304 for the information signal may be found. Thetarget value 305 is estimated by the first regulator 301 based on ablock error rate (BLER) target 308, which is set by the higher layers inthe mobile communication system, and an estimated BLER 309, which hasbeen estimated by a BLER estimator 310. The BLER estimator 310 bases itsestimation on CRC (Cyclic Redundancy Check) error bits on each block ofdata bits. The receiver processes the CRC error bits and forms a CRCerror flag 311. If this flag is in a state of “not set”, the block ofdata bits is assumed to be possible to recover correctly in thereceiver. Otherwise, if the flag is “set” the whole block of data bitsis considered in error. In WCDMA both the inner power control loop andthe outer power control loop is supported in both uplink and downlink.The inner power control loop is a fast control loop, with a 1.5 kHzfrequency, which is able to compensate for fast fading effects in thewireless propagation channel.

The SIR estimator 306 is estimating SIR for every slot of data, which isfor example every 10/15 ms for WCDMA. It is the BLER estimator 310,which is the time limiting factor of the power control unit 300. Itestimates the BLER by checking the CRC for every block of data bits andthis gives a very long delay in the system, since the second regulator302 cannot start its operation until it is given the target value 305.For example, for a block length of 20 ms and if 1000 blocks are neededto estimate the BLER, which would be reasonable for a BLER target 308 of0.01, this computation requires 20 seconds in time, which results in avery slow system. This is highly undesirable since the conditions of themobile propagation channel are constantly changing and it is importantthat the power control is working rapidly to be valid.

Thus the block diagram in FIG. 3 may not be sufficient or lack inperformance for systems, like WCDMA, that also employ high qualityservices, which have even higher demand for correct and fast powercontrol than for example speech services. Instead of only relying uponthe CRC, the quality measure may be supplemented with an estimation ofe.g. soft information. Such information may be an estimated uncoded biterror rate (ucBER) before a channel decoder in a receiver, also calledraw BER. Other examples of soft information are bit error rates or blockerror rates after an intermediate decoding iteration in the receiver, orreceived SIR. Using soft information may give a faster indication on thechanges in the wireless propagation channel. To estimate softinformation is briefly indicated in chapter 9 of “WCDMA for UMTS, RadioAccess for Third Generation Mobile Communications”, edited by HarriHolma and Antti Toskala, and printed by John Wiley and Sons, 2000.

SUMMARY OF THE INVENTION

It is a purpose of the present invention to provide a method ofdetermining a target value for a control loop of a power control systemwhich provides a statistically reliable target value within a shorttime.

The purpose above is achieved by method of determining a target valuefor a control loop of power control system, a power control system, anarrangement for determining such a target value, a power control unitcomprising such an arrangement, a transceiver comprising such a powercontrol unit, and a mobile communication system, which are shown tocomprise the features in the following independent claims. The preferredembodiments of the power control unit, method and system are apparentfrom the dependent claims.

To accumulate reliable statistics for a block error rate (BLER) isrecognized to be time consuming and is thus a limiting factor in a powercontrol system. The present invention realizes that a faster tuning forthe power control system instead could be achieved if the sent power isadjusted to, for example, a given target uncoded bit error rate (ucBER),which is a measure of soft information. The target uncoded bit errorrate is in turn adjusted depending on the BLER. Preferably, the outerpower control loop can be implemented with low complexity when usingsoft information in the form of ucBER. The ucBER is preferably estimatedfrom received pilot bits instead of from data bits carrying the realinformation. This estimation has the advantage that the pilot bits areknown bits, thus making them suitable for estimating an impact on thebits by a wireless propagation channel. Usually there is a differentnumber of data bits compared to the number of pilot bits in a slot.According to the invention, it has been realised that the statisticalreliability of the calculated bit error rate, and thus the resultingtarget value for the control loop, is improved when an underflow of theestimated symbols at the receiver is compensated for. Such an underflowmay be caused by transmission errors and the limited bit resolution ofthe estimated symbols at the receiver. This underflow is overcome by thepresent invention by underflow compensation.

The pilot bits are encoded as symbols each presenting a number of bits.At the receiver, an estimate of the transmitted pilot symbols isdetermined from the received pilot symbols and from an estimate of thetransmission channel. If the true transmission channel was known, thetransmitted pilot symbols could completely be recovered at the receiver.In reality, however, bit errors occur. An accumulated bit error isdetermined by comparing the estimated transmitted symbols with the knownreference pilot symbols and by counting the number of bits which areincorrectly recovered from the estimated transmitted pilot symbols.Since the estimated transmitted symbols are represented with a finitebit resolution, e.g. 4 bits, an underflow situation may occur where theestimated transmitted symbol is determined to be zero, thereby notcarrying sufficient information about the received bits. According tothe invention, such an underflow is compensated for in the calculationof the accumulated bit error.

It is an advantage of the present invention that it provides a lowcomplexity and realisable power control system, in which an outer powercontrol loop is based on soft information estimation to set a correcttarget value, i.e. a SIR reference value, for an inner power controlloop, which will make the power control system less time consuming thanthe prior art methods.

It shall be emphasized that the terms “comprises” and “comprising” whenused in this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other features, integers, steps,components or groups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be acquiredby reference to the following detailed description when taken inconjunction with the accompanying drawings wherein:

FIG. 1 illustrates a general prior art outer power control loop method.

FIG. 2 illustrates an example of a WCDMA system with uplink powercontrol.

FIG. 3 is an illustration of a prior art power control unit based oncyclic redundancy check (CRC) only.

FIG. 4 is an illustration of a cascaded power control unit based on softinformation and CRC according to a first embodiment of the presentinvention.

FIG. 5 is an illustration of a cascaded power control unit based on softinformation and CRC according to a second embodiment of the presentinvention.

FIG. 6 is an illustration of a power control system according to thepresent invention.

FIG. 7 is a flowchart illustrating a low-complexity method of estimatingthe ucBER according to the second embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The main purpose of power control in a mobile communication system is tocontrol the sent power, in such a way that a block error rate (BLER) ata receiver is held constant at a given target value. Since it takes along time to accumulate reliable statistics for the BLER, a fastertuning could be achieved if the sent power is adjusted to a given targetuncoded bit error rate (ucBER), which is a measure of soft information.The target uncoded bit error rate is in turn adjusted depending on theblock error rate. According to the embodiments of the present inventionthe outer power control loop can be implemented with low complexity whenusing soft information in the form of ucBER.

In FIG. 4 a schematic block diagram is shown for a power control unit400 for either uplink or downlink in a mobile communication system,where the power control unit 400 is using soft information according toa first embodiment of the present invention. The system comprises threeregulators in cascade and each of the regulators may for example be aPI-regulator, although other well-known regulators may be used, such asPID regulators. Also different types of regulators may be used for eachregulator in the power control unit 400. The third regulator 401determines the TPC command 402, which is comprised in the inner powercontrol loop. The inner power control loop sends the TPC command 402 toa transmitter (not shown) to inform how it should adjust its transmittedpower level. The power level is adjusted in a predetermined manner. Theadjustment is dependent on an estimated quality measure 403 compared tothe target value 406. If the estimated quality measure 403 is below thetarget value 406, a TPC command 402 is sent to the transmitter toincrease its power level, and if the estimated quality measure 403 isabove the target value 406, a TPC command 402 is sent to the transmitterto decrease its power. The third regulator 401 regulates on theestimated quality measure 403, which may be a SIR estimate, estimated bythe SIR estimator 404 on received bits, e.g. pilot bits 405. Preferably,this is performed every slot, which is 10/15 ms for WCDMA. The estimatedquality measure 403 is regulated to meet the SIR reference value 406,the target value, which is estimated by a second regulator 407. Thesecond regulator 407 and a first regulator 408 are arranged in cascadeand are comprised in the outer power control loop element 417. The firstregulator 408 has the same function as the first regulator 301 in theprior art power control unit 300 of FIG. 3. It regulates on theestimated BLER value 409 from a BLER estimator 410 to meet the BLERtarget value 411, set by higher layers of the system. The BLER estimator410 uses a CRC error flag 412 to determine errors in received blocks ofdata. The BLER estimation is based on a CRC error flag 412 for exampleby filtering the CRC error flag 412 with appropriate filter constants.Alternatively, the BLER estimator 410 uses a moving average to get theestimated BLER value 409 from the CRC error flag 412.

The result of the regulation in the first regulator 408 is an uncodedBER (ucBER) reference value 413 for the second regulator 407 to aim at.This second regulator 407 regulates an estimated ucBER 414 to meet theucBER reference value 413 and thereby estimate the SIR reference value406, or the target value for the inner power control loop 401. An ucBERestimator 415 forms the ucBER estimate 414, the soft information, basedon received data bits 416 and the CRC error flag 412. If the CRC errorflag 412 is in a state of being “set”, all the data bits are assumed tobe incorrect and retransmission is needed. However, if the flag 412 is“not set” this implies that raw data bits will be possible to recovercorrectly in the receiver. Then the ucBER estimator performs all theactions performed by the transmitter on the information signal, i.e. thecorrectly recovered data bits are for example coded and interleaved toreconstruct the correct raw data bits. The correct raw data bits arethen compared to the received data bits 416 to provide the ucBERestimate 414. The ucBER estimator 415 performs its operation preferablyon every block, which is for WCDMA every 30 slots. This means that itneeds 30×10/15 ms=20 ms in operational time. By adding the ucBERestimator 415 and the second regulator 407 in the outer power controlloop element 417, a quality measure for the wireless propagation channelis thus provided earlier than errors are visible by the CRC check andthe BLER estimation. It is therefore possible to speed up theperformance of the outer power control loop, since it is possible to geta first SIR reference value 406 before the first regulator 408 hascompleted its task. In this way the inner power control loop may startregulating earlier than the prior art methods. Though this firstembodiment is well functioning, it is computationally complex andrequires much hardware to estimate the ucBER, since it requires the samehardware as the transmitter to replicate its function on the data bits.

In a second embodiment of the present invention received pilot bits areused instead of the data bits to estimate the ucBER. A cascaded powercontrol unit 500 as illustrated in FIG. 5 describes the poweradjustment.

FIG. 5 illustrates a schematic block diagram for a power control unit500 for uplink or downlink in a mobile communication system, where thepower control unit 500 is using soft information and comprises threeregulators in cascade. Each regulator may for example be a PI-regulator,although other well-known regulators may be used, such as PIDregulators. Also different types of regulators may be used for eachregulator in the power control unit 500. An outer power control loopelement 516 comprises a first regulator 501 and a second regulator 502and an inner power control loop comprises a third regulator 503 whichproduces a transmit power control (TPC) command 504 to be sent to atransmitter (not shown) with information how the transmitter shouldadjust its power. The third regulator 503 regulates an estimated qualitymeasure 505, which may be an estimated signal-to-interference (SIR)value to meet a SIR reference value or target value 506, thereby formingthe TPC command 504. If the estimated quality measure 505 is larger thanthe target value 506 the TPC command 504 contains information to thetransmitter (not shown) to decrease its transmitted power level, and ifthe estimated quality measure 505 is above the target value 506, a TPCcommand 504 is sent to the transmitter to decrease its power level. ASIR estimator 507 estimates the estimated SIR value 505 on received bits508, e.g. pilot bits. Preferably, this is performed every slot, which is10/15 ms for WCDMA According to the second embodiment of the presentinvention, the received pilot bits 508 are also input to an ucBERestimator 509 which is of relatively low complexity. The ucBER estimator509 may operate according to the method presented below and withreference also to FIG. 7. An estimated ucBER 510, which is the bit errorrate (BER) before channel decoding in the receiver, is input to thesecond regulator 502, which regulates to meet a ucBER reference 511,thus producing the SIR reference value 506 to the third regulator 503.The ucBER estimator 509 performs its operation preferably on everyblock, which is for WCDMA every 30 slots. This means that it needs30×10/15 ms=20 ms in operational time. By adding the ucBER estimator 509and the second regulator 502 in the outer power control loop element516, a quality measure for the wireless propagation channel is thusprovided earlier than errors may be visible by the CRC check and theBLER estimation. It is therefore possible to speed up the performance ofthe outer power control loop, since it is possible to get a first SIRreference value 506 before the first regulator 501 has completed itstask. In this way the inner power control loop may start regulatingearlier than the prior art methods. The first regulator 501 regulates anestimated BLER value 512 from a BLER estimator 513 to meet a BLER targetvalue 514, which is set by higher layers in the mobile communicationsystem. The BLER estimation is based on a CRC error flag 515 for exampleby filtering the CRC error flag with appropriate filter constants.Alternatively, the BLER estimator 513 uses a moving average to get theestimated BLER value 512 from the CRC error flag 515.

The second embodiment of the present invention, further presents amethod to estimate the ucBER by using pilot bits instead of data bits,thereby making the ucBER estimator 509 of low complexity. The SIRreference value 506 is set by an outer closed control loop element 516,which acts as a cascaded control system, and the SIR reference value 506is used as target value for the inner closed power control loop 503. Theouter power control loop measures the quality of the received signal,i.e. how the propagation channel has affected the transmitted signal byestimating the uncoded, or raw, BER (ucBER) on received pilot bits 508.The pilot bits are known and since the received pilot bits haveexperienced the same or similar propagation conditions in the wirelesspropagation channel, the ucBER is substantially proportional to theucBER for the data bits. Although the pilot bits and the data bits maybe sent with different power levels, their relative power difference isconstant. This implies that the ucBER for the pilot bits follow theucBER for the data bits when the bits are sent on the same wirelesspropagation channel. As well known in the art of regulation, a constantrelative difference can always be compensated for by a closed controlloop.

In FIG. 6 illustrates a power control system 600 operating as a closedpower control loop in none direction in a mobile communication system.The power control system 600 comprises at least a first transceiver 601and a second transceiver 602. They communicate with each other over awireless propagation channel 603 with radio signals carrying differentkinds information. The first transceiver 601 comprises at least a firstreceiver 604, at least a first transmitter 605 and at least one powercontrol unit 606, which for example may be any of the power controlunits 400; 500 described above in relation to FIG. 4 and FIG. 5. Thesecond transceiver 602 comprises at least a second receiver 607 and atleast a second transmitter 608. If the first transceiver 601 is a basestation and/or an RCE in the mobile communication system, the powercontrol system 600 is employed for the uplink propagation direction, andvice versa if the first transceiver 601 is a mobile unit in the mobilecommunication system, the power control system 600 is employed for thedownlink propagation direction. However, the power control system may beemployed in both the uplink and the downlink. In this case the secondtransceiver would also comprise a second power control unit. However,only in the purpose of simplifying the description of the power controlsystem 600, the power control unit 606 is in FIG. 6 only located in thefirst transceiver 601. An information signal 609 is transmitted from thesecond transmitter 608 over the wireless propagation channel 603, whichaffects the signal in a random and unknown manner before received by thefirst receiver 604. The first receiver 604 processes the signal, by forexample despreading, decoding and deinterleaving and forms a newreceived signal 610, which is input to the power control unit 606, whichproduces a transmit power control command (TPC) 611. The TPC command 611is processed by the first transmitter 605 to form a radio signal 612carrying the TPC information for transmission over the wirelesspropagation channel 603. The TPC radio signal 612 is received andprocessed by the second receiver 607 to form a reconstructed TPC command613. The reconstructed TPC command 613 is input to control the powerlevel of the second transmitter 608 since the TPC command 613 containsinformation how to adjust the power level of the transmitted signal 609depending on the conditions of the wireless propagation channel 603.

In the second embodiment of the present invention the ucBER estimator509 may operate according to the following method, with reference toFIG. 7.

A receiver in a WCDMA system receives despreads symbols, y_(i,f), from aRake receiver, each symbol representing a number of bits, e.g. two bits.Here, i is the symbol index and f is the finger index of the Rake. Inone WCDMA slot there are 2560/sf symbols (pairs of bits) or 2×2560/sfbits, where sf is the spreading factor. Theny _(i,f) =h _(i,f) ·x _(i)+noise  (1)where h_(i,f) is a true propagation channel response, which models thewireless propagation channel's influence on the signal, and x_(i)represents the transmitted symbol representing complex transmitted bitinformation, i.e. xi=±1+i(±1). The noise consists of interference fromother base stations, quantization errors, and thermal noise. Thereceiver tries to estimate the channel response, h_(i,f). The estimationis denoted ĥ_(i,f).

Let w_(f) be a real valued weight factor that is inversely proportionalto an estimated noise power for finger f in the Rake receiver. Weightedchannel estimates w_(f)·ĥ_(i,f) are formed to compensate for the noise.They are then used with the despread symbols, y_(i,f), to estimate thetransmitted symbol, x_(i). Let the estimated transmitted symbol bedenoted {circumflex over (x)}_(i), then

$\begin{matrix}{{{\hat{x}}_{i} = {\sum\limits_{f = 1}^{N_{fingers}}\;{y_{i,f}\left( {w_{f} \cdot {\hat{h}}_{i,f}} \right)}^{*}}},} & (2)\end{matrix}$where N_(fingers) denotes the number of fingers used in the Rakereceiver and (.)* is the complex conjugate of (.). Substituting (1) into(2) and omitting the noise results in

$\begin{matrix}{{{\hat{x}}_{i} \approx {\left( {\sum\limits_{f = 1}^{N_{fingers}}\;{h_{i,f}\left( {w_{f} \cdot {\hat{h}}_{i,f}} \right)}^{*}} \right)x_{i}}},} & (3)\end{matrix}$which shows that if the quantity in parenthesis is a positive, realnumber, {circumflex over (x)}_(i) is proportional to x_(i) and thus isan estimate of the transmitted symbol. This is true for good channelestimates, ĥ_(i,f) and this means that the sent symbol has beenrecovered.

The uncoded bit error rate is defined as the bit error rate after harddecisions of the symbols {circumflex over (x)}_(i) in (2). Harddecisions mean that sign information (sgn) of the real and imaginaryparts of {circumflex over (x)}_(i) is extracted and compared to the realand imaginary parts of x_(i), respectively. Put algebraically, the rateof occurrences ofRe x _(i≠sgn() Re {circumflex over (x)} _(i))Im x _(i≠sgn() Im {circumflex over (x)} _(i))  (4)is counted to get the ucBER.

Let there be N_(ch) _(—) _(est) channel estimates per slot. In adedicated physical channel in WCDMA, the last symbols in a slot are thepilot symbols representing the pilot bits. As opposed to the data bits,which contain the actual information, the pilot bits are dummy bits,which are known by both the transmitter and the receiver. The ideaaccording to the second embodiment A present invention behind estimatingthe uncoded bit error rate, is to replace despread data symbols y_(i,f)in (2) by received despread dedicated pilot symbols, z_(i,f). Since thepilots are the last symbols in the slot, the calculation uses the lastweighted channel estimate, that is, wf·ĥ_(Nch) _(—) _(est,f). Then thechannel estimates based on the pilot symbols are defined as:

$\begin{matrix}{{{\hat{z}}_{i} = {\sum\limits_{f = 1}^{N_{fingers}}\;{z_{i,f}\left( {{w_{f} \cdot {\hat{h}}_{Nch\_ est}},f} \right)}^{*}}},} & (5)\end{matrix}$for symbol i and where z_(i,f) are the received despread pilot symbols.Since it is always known which pilot symbols, p_(i), are sent, thenumber of bit errors may be counted. The pilot and data bits belong tothe same physical channel and the wireless propagation channel willtherefore influence them in a similar manner. Hence, preferably, thechannel estimates used for the pilot symbols are derived in the same wayas the channel estimates used for the data symbols. In one embodiment,the channel estimates are based on the common pilot channel (CPICH).

It is noted that, alternatively to using the last channel estimate in aslot, another channel estimate may be used, preferably another channelestimate for that slot.

Due to limited bit-widths of the pilot symbols, {circumflex over(z)}_(i) may experience underflow, that is, it becomes zero and does notcontain any information. This effect is compensated for according to theinvention. In one embodiment, the {circumflex over (z)}_(i) arerepresented as signed 4 bit integers.

The ucBER estimator 509 uses the received pilot bits in the receivedpilot symbols, z_(i,f) 508 to estimate the ucBER 510. The flowchart inFIG. 7 shows a method according to the second embodiment of the presentinvention to estimate the ucBER, which comprises the following steps:

(i) Initialization 701 where a number of slots, N_(slot) that will beused in computing an estimate for the bit error rate is chosen and anumber of received pilot symbols, N_(pilot) that will be used in eachslot is chosen. Then a total of 2 N_(slot) N_(pilot) bits will be usedto estimate the bit error rate. Initially an accumulated number of biterrors, N_(ber), is set to zero.

(ii) Estimation 702 in each slot of the received despread pilot symbolsz_(i,f) 703 according to equation (5).

(iii) In each slot, calculation 704 of the accumulated number of biterrors, N_(ber), for the N_(pilot), received pilot symbols. The signinformation (sgn) of the real and imaginary parts of {circumflex over(z)}_(i) is extracted and compared to the real and imaginary parts ofthe known pilot symbols, p_(i) 705. Thus, the following calculations areperformed:

$\begin{matrix}{N_{ber} = \left\{ {\begin{matrix}{N_{ber},} & {{{Re}\mspace{14mu} p_{i}} = {{sgn}\;\left( {{Re}\mspace{11mu}{\hat{z}}_{i}} \right)}} & {{\hat{z}}_{i} \neq 0} \\{{N_{ber} + 1},} & {{{Re}\mspace{14mu} p_{i}} \neq {{sgn}\mspace{11mu}\left( {{Re}\mspace{11mu}{\hat{z}}_{i}} \right)}} & {{{\hat{z}}_{i} \neq 0},} \\{{N_{ber} + 0.5},} & {{\hat{z}}_{i} = 0} & \;\end{matrix}{and}} \right.} & (6) \\{N_{ber} = \left\{ \begin{matrix}{N_{ber},} & {{{{Im}{\mspace{11mu}\;}p_{i}} = {{sgn}\;\left( {{Im}\mspace{14mu}{\hat{z}}_{i}} \right)}},} & {{\hat{z}}_{i} \neq 0} \\{{N_{ber} + 1},} & {{{{Im}\mspace{14mu} p_{i}} \neq {{sgn}\;\left( {{Im}\mspace{14mu}{\hat{z}}_{i}} \right)}},} & {{{\hat{z}}_{i} \neq 0},} \\{{N_{ber} + 0.5},} & {{\hat{z}}_{i} = 0} & \;\end{matrix} \right.} & (7)\end{matrix}$

Here, p_(i) is the known transmitted pilot symbol 705 and {circumflexover (z)}_(i) is the estimated channel response for the receiveddespread pilot symbols {circumflex over (z)}_(i,f) 703.

Underflow compensation 706 is needed when {circumflex over (z)}_(i=)0.Then N_(ber)+0.5 is suitable for the accumulated bit error. To motivatethis, assume that for the transmitted data symbol stream, x_(i), thereal and imaginary parts of the data symbols take on the values +1 or −1with equal probability, i.e. P(Re x_(i)=1)=P(Im x_(i)=1)=P(Rex_(i)=−1)=P(x_(i)=−1)=0.5, where P(.) denotes probability. This im theconditional probabilities:P(Re x _(i)=1|{circumflex over (x)}_(i)=0)=P(Im x _(i)=1|{circumflexover (x)}_(i)=0

In other words, when {circumflex over (x)}_(i)=0, then half of the timeRe x_(i)=1, which results in a bit error, and half of the time Rex_(i)=−1, which does not result in a bit error. In (iii) this is modeledby adding 0.5, half an error, to the accumulated number of bit errors,N_(ber).

(iv) At the end of N_(slot)slots, computation 707 of an estimated biterror rate is performed byucBER=^(Nber)/_(2NslotNpilot)  (8)

(v) Finally, the ucBER estimate is input to the outer loop regulator forproducing a target value to the inner power control loop.

In an alternative embodiment for step (iii), in each slot, thecalculation 704 of the accumulated number of bit errors, N_(ber), forthe N_(pilot) received pilot symbols is performed according to:

$\begin{matrix}{N_{ber} = \left\{ {\begin{matrix}{N_{ber},} & {{{{Re}\mspace{14mu} p_{i}} = {{sgn}\left( {{Re}{\;\;}{\hat{z}}_{i}} \right)}},} & {{{Re}\mspace{14mu}{\hat{z}}_{i}} \neq 0} \\{{N_{ber} + 1},} & {{{{Re}\mspace{14mu} p_{i}} \neq {{sgn}\left( {{Re}{\hat{\mspace{11mu} z}}_{i}} \right)}},} & {{{{Re}\mspace{14mu}{\hat{z}}_{i}} \neq 0},} \\{{N_{ber} + 0.5},} & {{{Re}{\hat{\; z}}_{i}} = 0} & \;\end{matrix}{and}\begin{matrix}{N_{ber} = \left\{ \begin{matrix}{N_{ber},} & {{{{Im}\mspace{14mu} p_{i}} = {{sgn}\left( {{Im}{\;\;}{\hat{z}}_{i}} \right)}},} & {{{Im}\mspace{14mu}{\hat{z}}_{i}} \neq 0} \\{{N_{ber} + 1},} & {{{{Im}\mspace{14mu} p_{i}} \neq {{sgn}\left( {{Im}\mspace{14mu}{\hat{z}}_{i}} \right)}},} & {{{{Im}\mspace{14mu}{\hat{z}}_{i}} \neq 0},} \\{{N_{ber} + 0.5},} & {{{Im}\mspace{14mu}{\hat{z}}_{i}} = 0} & \;\end{matrix} \right.} & \left( {7a} \right)\end{matrix}} \right.} & \left( {6a} \right)\end{matrix}$

Hence, as in the above embodiment, the sign information (sgn) of thereal and imaginary parts of {circumflex over (z)}_(i) is extracted andcompared to the real and imaginary parts of the known pilot symbols,p_(i) 705. Here, p_(i) is the known transmitted pilot symbol 705 and{circumflex over (z)}_(i) is the estimated channel response for thereceived despread pilot symbols z_(i,f) 703.

According to this embodiment, underflow compensation 706 is performedwhen Re {circumflex over (z)}_(i)=0 and/or Im {circumflex over(z)}_(i)=0. If any of these situations arises, N_(ber)+0.5 is added tothe accumulated bit error.

It is noted that in the above embodiments, if the probabilities for +1and −1 are different than 0.5, a different correction factor may bechosen. For example, assuming that the probability for +1 is p and theprobability for −1 is (1−p) and assuming that Re(x_(i)) or Im(x_(i))being equal to zero is interpreted as +1, the correction factor ispreferably chosen to be 1−p.

In yet another alternative embodiment for step (iii), underflowcompensation is instead performed by letting the signs of the N_(pilot),symbols be changed in each slot, such that there are as many bits equalto +1 as bits equal to −1. The signs of the corresponding real andimaginary parts of {circumflex over (z)}_(i) then need be changed beforechecking for bit errors. The accumulated bit errors in step (iii) wouldthen be updated as:

$\begin{matrix}{N_{ber} = \left\{ {\begin{matrix}{N_{ber},} & {{{Re}\mspace{14mu} p_{i}} = {{sgn}\left( {{Re}{\;\mspace{11mu}}{\hat{z}}_{i}} \right)}} \\{{N_{ber} + 1},} & {{{Re}\mspace{14mu} p_{i}} \neq {{sgn}\left( {{Re}{\;\mspace{11mu}}{\hat{z}}_{i}} \right)}}\end{matrix}{and}\begin{matrix}{N_{ber} = \left\{ \begin{matrix}{N_{ber},} & {{{Im}\mspace{14mu} z_{i}} = {{sgn}\left( {{Im}\mspace{11mu}{\hat{z}}_{i}} \right)}} \\{{N_{ber} + 1},} & {{{Im}\mspace{14mu} z_{i}} = {{sgn}\left( {{Im}{\;\mspace{11mu}}{\hat{z}}_{i}} \right)}}\end{matrix} \right.} & (10)\end{matrix}} \right.} & (9)\end{matrix}$

In a preferred embodiment, the above change in sign is performed inaddition to the underflow compensation according to step (iii), therebyensuring that there are as many bits equal to +1 as bits equal to −1.Hence it is ensured that the probabilities are P(Re p_(i)=1)=P(Imp_(i)=1)=P(Re p_(i)=−1)=P(Im p_(i)=−1)=0.5, and the accuracy of theunderflow compensation is improved.

According to yet another embodiment of the present invention a morecomplex approach would be to add a step of estimating a relative powerdifference, α, between the received pilot symbols and the received datasymbols and then multiplying each of the estimated pilot symbols{circumflex over (z)}_(i) by the relative power difference, a. This stepwould be performed before step (iii) or step (iv). The estimation of amay be performed by filtering the power of a restricted number of pilotand data symbols. Alternatively, the mobile communications network mayprovide the value of α.

Although preferred embodiments of the method and the system of thepresent invention have been illustrated in the accompanying drawings anddescribed in the foregoing detailed description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims. For example, while the embodiments ofthe invention have been described with respect to WCDMA, the inventionis not limited thereto but may certainly be applicable to other mobilecommunication systems and combinations of different mobile communicationsystems.

The term “transceiver” used in this specification includes various kindsof mobile communication units present in a mobile communication system.Also the present invention is not limited to single-band or single-modetransceivers, but includes transceivers serving at least one mobilecommunication system.

The term “mobile unit” used in this specification includes various kindsof portable or mobile radio communication equipment, such as mobiletelephones, pagers, electronic organizers, smart phones, communicatorsand other portable communication apparatus.

1. A method of determining a target value for a control loop of a powercontrol system in a mobile radio communications system, the methodcomprising: receiving a communications signal via a communicationschannel, the communications signal representing a number of receiveddata symbols and a number of received pilot symbols; determining anumber of estimated transmitted pilot symbols from the number ofreceived pilot symbols and from a channel estimate of the communicationschannel; calculating an accumulated pilot bit error from the number ofestimated transmitted pilot symbols and from a set of reference pilotsymbols, wherein the accumulated pilot bit error is compensated for anunderflow in the estimated transmitted pilot symbols, if at least afirst component of the estimated transmitted pilot symbol is equal tozero, and the compensation comprises the step of adding half an error tothe accumulated pilot bit error; and determining the target value on thebasis of the calculated pilot bit error.
 2. The method according toclaim 1, in which the first component of the first reference pilotsymbol is one of the real and imaginary parts of the first referencepilot symbol, and wherein the first component of the first estimatedtransmitted pilot symbol is one of the real and imaginary parts of thefirst estimated transmitted pilot symbol.
 3. The method according toclaim 1, further comprising dividing the accumulated pilot bit errorwith the total number of bits to obtain an uncoded bit error rate. 4.The method according to claim 1, in which the control loop is an innerpower control loop and the target value is determined by an outercontrol loop on the basis of the calculated pilot bit error and a targetbit error rate.
 5. The method according to claim 4, in which the outerpower control loop and the inner power control loop are closed powercontrol loops.
 6. The method according to claim 1, in which the targetvalue represents a signal-to-interference reference value.
 7. The methodaccording to claim 1, in which the method is employed on an uplinkpropagation direction in the mobile radio communications system.
 8. Themethod according to claim 1, in which the method is employed on adownlink propagation direction in the mobile radio communicationssystem.
 9. The method according to claim 1, in which the reference pilotsymbols represent reference pilot symbols of a WCDMA system.
 10. Themethod according to claim 1, wherein each of the estimated transmittedpilot symbols is calculated from an estimated channel response of thecorresponding reference pilot symbol.
 11. A method of determining atarget value for a control loop of a power control system in a mobileradio communications system, the method comprising: receiving acommunications signal via a communications channel, the communicationssignal representing a number of received data symbols and a number ofreceived pilot symbols; determining a number of estimated transmittedpilot symbols from the number of received pilot symbols and from achannel estimate of the communications channel; calculating anaccumulated pilot bit error from the number of estimated transmittedpilot symbols and from a set of reference pilot symbols, wherein theaccumulated pilot bit error is compensated for an underflow in theestimated transmitted pilot symbols, wherein the step of calculating theaccumulated pilot bit error further comprises: incrementing theaccumulated pilot bit error by a compensation factor, if a firstcomponent of a first one of the number of estimated transmitted pilotsymbols is equal to zero; otherwise incrementing the accumulated pilotbit error based on a comparison of the first component of the firstestimated transmitted pilot symbol with a corresponding first componentof a corresponding first one of the set of reference pilot symbols, anddetermining the target value on the basis of the calculated pilot biterror.
 12. The method according to claim 11, in which the firstcomponent of the first reference pilot symbol is one of the real andimaginary parts of the first reference pilot symbol, and wherein thefirst component of the first estimated transmitted pilot symbol is oneof the real and imaginary parts of the first estimated transmitted pilotsymbol.
 13. A method of determining a target value for a control loop ofa power control system in a mobile radio communications system, themethod comprising: receiving a communications signal via acommunications channel, the communications signal representing a numberof received data symbols and a number of received pilot symbols;determining a number of estimated transmitted pilot symbols from thenumber of received pilot symbols and from a channel estimate of thecommunications channel; calculating an accumulated pilot bit error fromthe number of estimated transmitted pilot symbols and from a set ofreference pilot symbols, wherein the accumulated pilot bit error iscompensated for an underflow in the estimated transmitted pilot symbols;wherein the step of calculating an accumulated pilot bit error isperformed by hard decisions according to: $\begin{matrix}{N_{ber} = \left\{ {\begin{matrix}{N_{ber},} & {{{Re}\mspace{14mu} p_{i}} = {{sgn}\;\left( {{Re}\mspace{11mu}{\hat{z}}_{i}} \right)}} & {{{Re}\mspace{14mu}{\hat{z}}_{i}} \neq 0} \\{{N_{ber} + 1},} & {{{Re}\mspace{14mu} p_{i}} \neq {{sgn}\;\left( {{Re}\mspace{11mu}{\hat{z}}_{i}} \right)}} & {{{Re}{\hat{\mspace{11mu} z}}_{i}} \neq 0} \\{{N_{ber} + 0.5},} & {{{Re}\mspace{11mu}{\hat{\; z}}_{i}} = 0} & \;\end{matrix}{and}} \right.} \\{N_{ber} = \left\{ \begin{matrix}{N_{ber},} & {{{{Im}{\mspace{11mu}\;}p_{i}} = {{sgn}\;\left( {{Im}\mspace{14mu}{\hat{z}}_{i}} \right)}},} & {{{Im}\mspace{14mu}{\hat{z}}_{i}} \neq 0} \\{{N_{ber} + 1},} & {{{{Im}\mspace{14mu} p_{i}} \neq {{sgn}\;\left( {{Im}\mspace{14mu}{\hat{z}}_{i}} \right)}},} & {{{Im}\mspace{14mu}{\hat{z}}_{i}} \neq 0} \\{{N_{ber} + 0.5},} & {{{{Im}\mspace{20mu}{\hat{z}}_{i}} = 0},} & \;\end{matrix} \right.}\end{matrix}$ where ^(Nber) is the accumulated pilot bit error, pi is areference pilot symbol, ^({circumflex over (z)}i) is the estimatedtransmitted pilot symbol, Re denotes the real part of a symbol, Imdenotes the imaginary part of a symbol, and sgn is the sign operator,and determining the target value on the basis of the calculated pilotbit error.
 14. A method of determining a target value for a control loopof a power control system in a mobile radio communications system, themethod comprising: initializing a power control system, which compriseschoosing a total number of bits to be used in calculating theaccumulated bit error, the total number of bits being 2 N_(slot)N_(pilot), where N_(slot) is a number of slots to be used in calculatingthe accumulated bit error, and N_(pilot), is a number of received pilotsymbols to be used for every slot, receiving a communications signal viaa communications channel, the communications signal representing anumber of received data symbols and a number of received pilot symbols;determining a number of estimated transmitted pilot symbols from thenumber of received pilot symbols and from a channel estimate of thecommunications channel; calculating an accumulated pilot bit error fromthe number of estimated transmitted pilot symbols and from a set ofreference pilot symbols, wherein the accumulated pilot bit error iscompensated for an underflow in the estimated transmitted pilot symbols;and determining the target value on the basis of the calculated pilotbit error.
 15. A method of determining a target value for a control loopof a power control system in a mobile radio communications system, themethod comprising: receiving a communications signal via acommunications channel, the communications signal representing a numberof received data symbols and a number of received pilot symbols;determining a number of estimated transmitted pilot symbols from thenumber of received pilot symbols and from a channel estimate of thecommunications channel; estimating a relative power difference, α,between the received pilot bits and the received data bits andmultiplying each of the estimated pilot symbols by the relative powerdifference, α; calculating an accumulated pilot bit error from thenumber of estimated transmitted pilot symbols and from a set ofreference pilot symbols, wherein the accumulated pilot bit error iscompensated for an underflow in the estimated transmitted pilot symbols;and determining the target value on the basis of the calculated pilotbit error.